Cosmetic compositions, such as sunscreens, self-tanning compositions, after-sun moisturizing compositions, and depilatories, produce a cosmetic, protective, moisturizing, softening, and/or soothing effect when applied to the human skin. Sunscreen compositions, for example, are applied to the skin to protect the skin from the sun's ultraviolet rays that produce erythema, a reddening of the skin commonly known as sunburn. Ultraviolet radiation in the wavelength range of 290 nm to 320 nm (“UV-B”), which is absorbed near the surface of the skin, is the primary cause of sunburn. Ultraviolet radiation in the wavelength of 320 nm to 400 nm (“UV-A”) penetrates more deeply into the skin and can cause damaging effects that are more long term in nature. Prolonged and constant exposure to the sun may lead to actinic keratoses and carcinomas as well as to premature aging of the skin, characterized by skin that is wrinkled, cracked, and has lost its elasticity.
Sunscreen agents, which can be divided into two classes, organic and inorganic, can be formulated into various formats of cosmetic products including creams, lotions, sticks, gels and sprays. A highly desirable method for delivering a cosmetic composition, such as a sunscreen composition, to the skin and hair is in the form of a finely dispersed spray. A finely dispersed spray produces improved coverage of the skin and hair and allows easier application to difficult to reach areas. Such a spray is desirably delivered using a non-aerosol spray pump, which does not require the use of pressurized containers or special aerosolizing gases.
The ability of pump-driven delivery systems to deliver a cosmetic composition as a finely divided spray is critically dependent upon the rheology of the cosmetic composition, particularly its viscosity at the exit port of the spray pump. As the viscosity of the composition decreases at the exit port, the spray pattern becomes more divided and produces a more desirable delivery by evenly covering a large area. Conversely, as the viscosity increases, the spray pattern becomes less divided and more stream-like, yielding a less desirable delivery, either by covering only a small area or by unevenly covering a larger one.
The effectiveness of sunscreen compositions and other cosmetic compositions is influenced by their rheology under both high and low shear conditions (see, for example, Sunscreens. Development, Evaluation and Regulatory Aspects, N. J. Lowe, N. A. Shaath and M. A. Pathak, Eds, Marcel Dekker, 1997). Sunscreen compositions having low viscosity at high shear rates tend to be easy to spread on the skin and can produce more even coverage and, hence, higher sun protection factors. However, these compositions have a number of deficiencies. They tend to drip or run after application and thus need to be spread immediately after application. This undesirable tendency to run after application can result in the composition dripping into the eyes, especially the eyes of children, or dripping onto clothing. Furthermore, if the viscosity remains low after spreading, the compositions tend to run off the ridges of the skin and accumulate in wrinkles resulting in uneven protection and low sun protection factors.
To overcome these deficiencies, sunscreen compositions are often formulated to have high viscosities. However, high viscosity compositions are more difficult to spread evenly on the skin, resulting in reduced protection from ultraviolet radiation, and often have a heavy skin-feel in addition to the delivery problems encountered with the spray pattern.
Although both oil-in-water and water-in-oil emulsions are used as delivery vehicles, sprayable sunscreen compositions are typically oil-in-water emulsions because of advantages in terms of skin-feel, cost-in-use, and formulation convenience. Stable oil-in-water emulsions are difficult to prepare at very low viscosities. In addition, at very low viscosity it is difficult to achieve good long term suspension of inorganic sunscreen agents, such as titanium dioxide or zinc oxide, that reflect, scatter, and/or absorb ultraviolet radiation and prevents it from reaching the skin and hair. For the inorganic sunscreen to effectively block the ultraviolet radiation, it must be dispersed in the sunscreen composition, in either the oil phase or the aqueous phase.
To improve the water resistance of the sunscreen composition, an emulsion that is stable in the container but breaks down rapidly on shearing is formulated. When an oil-in-water emulsion is delivered to the skin, the water evaporates leaving an oil layer. If the original emulsion is stable to shear, the oil layer will be re-emulsified on wetting of the skin and will be washed off. However, if the original emulsion was unstable to shear, it will break down on spreading and will not be re-emulsified when the skin becomes wet and, thus, will remain on the skin. Typically, emulsions unstable to shear require formulation of an emulsion with a minimum level of emulsifier. This can produce storage stability problems.
High levels of sunscreen agents are required to produce sunscreen compositions with high sun protection factors. This creates formulation difficulties as the organic sunscreen agents must be solubilized and emulsified and the inorganic sunscreen agents must be dispersed and suspended in the sunscreen composition. In addition, all sunscreen agents are expensive.
Thus, a need exists for a sunscreen composition that remains a stable emulsion or suspension during storage in the container and yet has a sufficiently low viscosity when sheared so that it can be effectively delivered as a fine spray using a non-aerosol spray pump and will then resist the tendency to drip.